A federal judge in Florida struck down President Barack Obama’s landmark healthcare overhaul as unconstitutional on Monday, in the biggest legal challenge yet to federal authority to enact the law.

U.S. District Judge Roger Vinson, appointed to the bench by Republican President Ronald Reagan, ruled that the reform law’s so-called individual mandate went too far in requiring that Americans start buying health insurance in 2014 or pay a penalty.

“Because the individual mandate is unconstitutional and not severable, the entire act must be declared void. This has been a difficult decision to reach, and I am aware that it will have indeterminable implications,” Vinson wrote.